Binary input java example (#363)

## Binary Input Example
The goal of this example project is to test the performance of Engflow's
remote execution and caching service based on the number of input binary
files in the dependency graph. The project contains a `genrule` that
generates a specified number of Java binaries for the `genbinary` Java
library, which are then listed as dependencies in the main binary. The
`Main.java` file loops through each generated class and calls its
`greetNum` method.

### How It Works

1. **Generation of Java Binaries:**
- The `genrule` in the `BUILD` file generates a specified number of Java
classes (`Hello1.java`, `Hello2.java`, ..., `HelloN.java`).
- Each generated class contains a `greetNum` method that prints a unique
message.

2. **Main Class Execution:**
- The `Main.java` file in `binaryinput` dynamically loads each generated
class using reflection.
- It then invokes the `greetNum` method of each class, printing the
corresponding message.

---------

Co-authored-by: Andrés Felipe Barco Santa <[email protected]>
Co-authored-by: Andrés Felipe Barco <[email protected]>

Author: NicolasHuertas

java/com/engflow/binaryinput/BUILD

@@ -0,0 +1,32 @@
+load("@rules_java//java:defs.bzl", "java_binary", "java_library")
+
+NUM_FILES = 100
+
+# Generates a number of java files based on the value of NUM_FILES
+# Each file is named HelloX.java where X is the number of the file
+# Each file contains a class with a greetNum method that prints "Hello" + the number of the file
+[genrule(
+    name = "Hello" + str(x),
+    outs = ["Hello" + str(x) + ".java"],
+    cmd_bash = "echo 'package com.engflow.binaryinput;" + "\n" +
+    "public class Hello" + str(x) +
+    " { public static void greetNum() { System.out.println(\"Hello " + str(x) + "\"); } }' > $@",
+) for x in range(1,NUM_FILES+1)]
+
+# Generates a java library that contains all the generated java files
+[java_library(
+    name = "genbinary" + str(x),
+    srcs = [":Hello" + str(x) + ".java" for x in range(1,NUM_FILES+1)],
+    visibility = ["//visibility:public"],
+) for x in range(1,NUM_FILES+1)]
+
+# Main class
+java_binary(
+    name = "main",
+    srcs = ["Main.java"],
+    main_class = "com.engflow.binaryinput.Main",
+    deps = [
+        ":genbinary" + str(x) for x in range(1,NUM_FILES+1)
+    ],
+    args = [str(NUM_FILES)],
+)

java/com/engflow/binaryinput/Main.java

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+package com.engflow.binaryinput;
+
+import java.lang.reflect.InvocationTargetException;
+
+public class Main {
+    public static void main(String[] args) {
+        try {
+            // args[0] is the number of files to read
+            int numFiles = Integer.parseInt(args[0]);
+
+            // Load and run the greetNum method from each class
+            for(int i = 1; i <= numFiles; i++){
+                Class<?> clazz = Class.forName("com.engflow.binaryinput.Hello" + i);
+                clazz.getMethod("greetNum").invoke(null);
+            }
+
+        } catch (ClassNotFoundException | InvocationTargetException | IllegalAccessException | NoSuchMethodException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

java/com/engflow/binaryinput/README.md

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+# Multiple Binary Input Example
+
+## Usage
+Set the `NUM_FILES` variable in the BUILD file to the desired input size.
+
+To generate the test files, build the `genbinary` library using the `genrule`:
+```sh
+bazel build //java/com/engflow/binaryinput:genbinary{1..<NUM_FILES>}
+```
+
+Then, the program can be built with the following command:
+```sh
+bazel build //java/com/engflow/binaryinput:main
+```
+
+## How It Works
+
+1. **Generation of Java Binaries:**
+    - The `genrule` in the `BUILD` file generates a specified number of Java classes (`Hello1.java`, `Hello2.java`, ..., `HelloN.java`).
+    - Each generated class contains a `greetNum` method that prints a unique message.
+    - A java library is created for each file (`Hello1.jar`, `Hello2.jar`, ..., `HelloN.jar`).
+      
+2. **Building the main target:**
+   - The previously created libraries are added to the main class as dependencies through a for loop.
+   - The consistent naming scheme of the libraries simplifies their inclusion in the build process.
+
+3. **Main Class Execution:**
+    - The `Main.java` file in `binaryinput` dynamically loads each generated class using reflection.
+    - It then invokes the `greetNum` method of each class, printing the corresponding message.
+
+## Configuration
+
+The number of generated files is controlled by the `NUM_FILES` variable in the `BUILD` file of the `binaryinput` package. Modify this variable to change the number of generated classes and observe the performance impact on Engflow's remote execution and caching service.
+
+## Example
+
+To generate and run the program with 10 input binary files:
+
+1. Set `NUM_FILES` to 10 in `java/com/engflow/binaryinput/BUILD`.
+2. Build the `genbinary` library:
+   ```sh
+   bazel build //java/com/engflow/binaryinput:genbinary{1..10}
+   ```
+3. Build the `main` binary:
+   ```sh
+   bazel build //java/com/engflow/binaryinput:main
+   ```
+
+This will generate 10 Java classes, build the `genbinary` library, and build the `main` binary. Using `bazel run` will also print messages from each generated class.